1. Field of the Invention
The present invention relates to an improvement of an inverter system for inputting alternating current(AC) and direct current(DC) in combination, which can convert AC electric power, for instance, from a commercial AC power source to DC electric power and further convert this DC electric power to AC electric power to be supplied to a load and which can convert DC electric power, for instance, from a DC electric power source such as a solar cell to be fed to a load. More particularly, the invention relates to an inverter system for inputting alternating current and direct current in combination, which is adapted to effectively use both an AC electric power source and a DC electric power source when the AC electric power source and the DC electric power source are used in combination.
2. Related Art Statement
Recently, solar cells have been spotlighted as one of substituting energies. Needless to say, the solar cells are to convert a solar energy to an electic energy, and therefore largely influenced by natural conditions of an area where the solar cells are placed. Thus, it is often difficult to handle the solar cells as in the case of general commercial AC electric power sources and special consideration is necessary therefore. That is, although an energy source of the solar cells is almost inexhaustible and therefore their use value is extremely high, on the other hand, the solar cells are difficult to handle because the output thereof is exclusively influenced by natural environment and is very variable. As a result, it is a current situation that it is extremely difficult for the solar cells to independently supply general loads such as home electric lights with electric power. For this reason, when the solar cells are intended to be used as the electic power supply sources, it is effective and reasonable to use them in combination with general electric power supply sources.
On the other hand, since the solar cell functions as a DC electirc power source, it can not be directly connected to an AC system. Therefore, an inverter is necessary for converting DC electric power to AC electric power.
Incidentally, most of general electric power loads are AC operable loads. In addition various kinds of AC electric powers have different number of phases, different voltages, and different frequencies have recently been required depending upon the kinds of loads, for instance, as in the case that a single phase electric light load and a three-phase air conditioner which has lately been propagated are simultaneously used in a single home.
Accordingly, a variety of inverter systems have up to now been developed to cope with such electric power requirement. A generally known inverter system comprises an AC-DC converter for once converting an AC electric power from a commercial AC electric power source to a DC electric power and a DC-AC converter for converting this DC electric power to an AC electric power having a specific frequency, a specific voltage, and a specific number of phase, and is adapted to supply the thus converted AC electric power to a load.
However, according to a conventional technique, when a solar cell is used as an electric power supply source, a DC-AC converter exclusively used for converting a DC electric power to an AC electric power has been provided separately from an inverter system for AC electic power source.
However, since such a conventinal technique a large cost for installing the exclusive use DC-AC converter, it has been a great barrier against public propagation of solar cells.
Under the circumstances, there has been proposed an inverter system in which DC electric power can be converted to AC electric power by connecting a DC electiric power source to a DC-AC converter of the inverter system.
But, such a construction has a problem that although the DC electric power source is always connected to the inverter system, the DC electric power source is not available when the operation of a load in the inverter system is stepped down, so that effective utilization of the DC electric power source, more specifically, flexibility between the solar cells and the load can not be attained.